Electroless plating process

ABSTRACT

Improved deposition rates from electroless plating baths are described. The increase in deposition rate, which may be as great as seven-fold, arises from the use of a rotating substrate holder. Where uniformity of thickness is critical, periodic reversal of rotation may be used.

United States Patent; 1191 Okinaka et a1.

[11] 3,798,056 [451 Mar. 19, 1974 1 ELECTROLESS PLATING PROCESS [75] Inventors: Yutaka Okinaka, Madison; Richard Sard, Westfield, both of NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: Apr. 5, 1972 [21] Appl. No.: 241,363

[52] U.S. Cl. 117/113, 117/130 E, 118/409, 204/212 [51] Int. Cl B0lk 1/00, C23c 17/00 [58] Field of Search 117/160 R, 130 E, 130 R,

117/100 R,l13, 101, 213, 212; 118/54, 409, 116; 204/14 R, 43 R, 40 R, 212, 213

[56] References Cited UNITED STATES PATENTS 3,617,363 11/1971 Metzger 117/160 R 1 H1962 Leibouitz 204/212 3.063921 3.138.479 6/1964" Foley 117/160 R 3,692,638 9/1972 Rackus et a1. 204/212 Primary Examiner-Alfred L. Leavitt Assistant ExaminerMichael W. Ball Attorney, Agent, or FirmG. S. lndig [5 7] ABSTRACT Improved deposition rates from electroless plating baths are described. The increase in deposition rate, which may be as great as seven-fold, arises from the use of a rotating substrate holder. Where uniformity of thickness is critical, periodic reversal of rotation may I be used.

3 Claims, 2 Drawing Figures PATENTEDMAR 1 9 I914 3798056 FIG.

FIG. 2

RATE OF DEPOSITION, MICROMETERS/HR ES 1 l l l O 50 I00 I50 200 250 300 350 LINEAR VELOCITY OF ROTATION, Cm/SEC.

ELECTROILESS PLATIING PROCESS FIELD OF THE INVENTION The invention is concerned with deposition of metals by the electroless plating process, and, more particularly, with increasing that rate on miniature discrete piece parts, such as employed in integrated circuit technology.

BACKGROUND OF THE INVENTION The deposition of a metal onto a substrate is usually accomplished by one of three processes: electrolytic plating, vapor plating, and electroless plating. Electroless plating, as referred to in this application, deals with autocatalytic plating and is to be distinguished from displacement plating.

In the semiconductor industry, the deposition of gold onto silicon wafers to form electrodes in integrated circuit fabrication is usually accomplished by electrolytic plating; see M. P. Lepselter, U.S. Pat. No. 3,287,612, issued Nov. 22, 1966. Typically, contact is made to silicon in such devices by using a three-layer electrode structure, comprised of titanium, platinum or palladium, and gold. The titanium makes contact to selected regions of the underlying silicon. Initially, titanium and platinum (or palladium) cover the entire surface, so as to form a continuous cathodic surface, onto which the gold may be electrolytically deposited. Following deposition of the gold layer, the electrode patterns are formed by etching away undesired portions of the three-layer metallic structure.

It has long been recognized that an electroless process for depositing gold would be of great importance in reducing the number of steps now required. For example, the titanium and platinum or palladium layers could be vapor-deposited through masks in the desired electrode pattern. Then, electroless deposition of gold onto platinum or palladium could proceed autocatalytically. However, commercially available gold plating baths are known to be of the displacement type rather than of the autocatalytic type, thereby limiting the thickness of the plating. Recently, compositions for electroless (autocatalytic) gold plating have been disclosed; see Vol. 57, Plating, pages 914-920 (1970) and Vol.58, Plating, pages 1,080-1 ,084 1971 Such baths have proven useful, for example, in depositing gold onto integrated circuit chips to form beam lead electrodes.

It has been known for some time that agitation of electroless baths results in improved deposition; see, for example, Electroless Nickle Plating, edited by the American Society for Testing Materials (1959), page 29. Such methods, however, generally deal with mechanical stirring or forced circulation of the plating bath around a stationary piece. with little control over the rate of flow past the surface to be plated.

SUMMARY OF THE INVENTION In accordance with the invention, a method that increases the deposition rate of electroless plating solutions under controlled conditions is reported. This is accomplished by rotating the item to be plated at a linear velocity from 50 centimeters per second to 300 centimeters per second. Although examples are drawn to gold plating solutions, in which increases in deposition ratesof up to seven-fold are attained, the method is applicable to all autocatalytic plating procedures. In applications where uniformity of thickness is critical, periodic reversal of rotation is preferred.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1, which is a perspective view with some exploded sections, is an example of a rotating substrate holder, useful in the practice of the invention; and

FIG. 2, on coordinates of micrometers per hour and centimeters per second, is a plot of the variation in deposition rate with linear velocity of rotation.

DETAILED DESCRIPTION 1. Rotating Substrate Holder In FIG. 1, a rotating substrate holder 10, capable of holding eight silicon wafers 18, is shown. The wafer 18 shown here has a diameter of 3.08 centimeters and typically is divided into approximately 200 chips (as illustrated by the criss-cross lines), each chip having 32 beam leads for contacting to external circuitry. The beam leads, too small to be shown here, generally have dimensions of about 1,200 micrometers, onto which gold must be deposited to a thickness of about 10 micrometers to 12 micrometers. Thus, the great number of small, discrete regions to be plated will be appreciated.

The holder 10 consists of two wheels 11 and 12 man ufactured from an inert material and a shaft 15, comprised of an outer inert layer 14 protecting a stainless steel core 13. Both the upper wheel 11 and the lower wheel 12 have grooves 16 cut in an octagonal shape. The sloped notches 17 on the upper wheel 11 prevent hydrogen gas from accumulating on the underside of the wheel during plating. The silicon wafers 18 are mounted on smooth, inert substrates 19; an example of such a substrate is high density aluminum oxide (Al- O The substrates 19 are inserted in the grooves 16 of the lower wheel 12 in such a manner that the surfaces to be plated face outward. After the upper wheel 11 is positioned in place, screws 20, two each under the lower wheel 12 and above the upper wheel 11, are tightened to securely hold the substrates 119 between the wheels 11 and 12. The shaft 15 is then connected to a motor (not shown) for rotation of the holder 10. The material used for the wheels 11 and 12, the protective layer 14, and the screws 20 may be of any material inert in any of the common electroless plating baths known in the art. For example, a commercial preparation of polytetrafluoroethylene (Teflon, available from E. l. DuPont de Nemours & Co., Wilmington, Delaware) may be used.

For descriptive purposes only, a holder specifically designed for simultaneously plating eight silicon wafers has been shown here. In commercial production, larger holders, of whatever design, may be used to accommodate many more such wafers (or any other desired piece parts). In any event, it is contemplated that a multiplicity of such piece parts will be plated simultaneously, employing the rotation method described herein.

It is observed that by increasing the linear velocity of the holder, an initial increase in deposition rate results, followed by an approximate leveling off as the rotation velocity is further increased, and, finally, by a decrease in plating rate. Such a variation in deposition rate with increasing linear velocity is shown in FIG. 2. The curve, derived under the conditions described in Example (a) below, is approximately valid for the rotating holders contemplated for use in accordance with the invention. Assuming that a minimum of approximately a threefold increase in deposition rate is desirable, a preferred range of the linear velocity of rotation may be set at about 50 centimeters per second to 300 centimeters per second.

2. EXAMPLES The examples given below are drawn to the electroless deposition of gold. The gold plating baths are specific examples within the concentration range given in the 1970 Plating reference:

KAu(CN), 0.001M to 0.05M KCN 0.005M to 0.5M KOH 0.05M to IM KBH, 0.05M to IM The baths may be operated in the range of 60 C to 90 C.

Other substances in addition to those listed may be added, for example, to adjust pH, to increase the plating rate, and to improve the properties of the plated film for particular applications. In general, the gold deposited from these plating baths is at least 99 percent gold, with the balance being impurities incorporated in the deposited plating. The addition of other metal ions to the plating bath may also be made in order to deposit gold alloys. For example, metals such as cobalt, nickel, arsenic, copper, and silver are suitable for gold alloying. In general, such gold alloys so deposited consist of at least 90 percent gold.

a. A rotating substrate holder similar to that described above was used to study the variation in plating rate as a function of linear velocity of the holder. The particular holder employed had a radius of 1.75 centimeters. The gold plating bath had the following composition:

KAu(CN), 5.8 grams per liter (0.02M) KCN 13.0 grams per liter (0.2M) KOH 11.2 grams per liter (0.2M) KBH 21.6 grams per liter (0.4M)

The plating bath was maintained at a temperature of 75 d: 0.5 C by means of an external heating bath. Without rotation of the substrate holder, the plating rate of this bath was 0.7 micrometers per hour. Referring to FIG. 2, it can be seen that a deposition rate of about 3.0 micrometers per hour was achieved by rotation of the substrates at a linear velocity of about 250 centimeters per second.

b. A second rotating substrate holder, substantially as shown in FIG. 1, was also constructed. The substrate holder was designed to accommodate silicon wafers having a diameter of 3.08 centimeters. The holder had a radius of 4.76 centimeters. In this example, the gold plating bath had the following composition:

KAutCN), 0.86 grams per liter (0003M) KCN 6.5 grams per liter (0.1M) KOH l 1.2 grams per liter (0.2M) KBI-L 10.8 grams per liter (0.2M)

This bath was also operated at a temperature of 75 i 0.5" C. Without agitation, the plating rate-of this bath was 1 micrometer per hour. In addition, the physical properties of the deposit (e.g., porosity, appearance, etc.) were unsatisfactory for device use. However, by rotating the substrates at a linear velocity of about centimeters per second, a plating rate of about 7 micrometers per hour was obtained. The physical properties of the deposit were also improved, exhibiting qualities satisfactory for device use.

While the examples described herein have dealt with electroless gold plating baths, with specific application in semiconductor processing, the procedure described works for all other autocatalytic plating baths.

3. OTHER CONSIDERATIONS Rotation of the holder in one direction for the period of time necessary to deposit a desired thickness of gold on a surface has led to the observation that there is some edge build-up (that is, greater thickness) of the edges of the surface relative to the center region. In some cases, there may be as much as a 25 percent to 30 percent difference in thickness between edge and center. However, in many applications, this nonuniformity of thickness is not necessarily considered to be of serious consequence. On the other hand, there are applications in which uniformity of the thickness of the deposit may be important. Such nonuniformity in the thickness may be substantially reduced by a periodic reversal of the substrate holder.

It is contemplated that the conditions employed in periodic reversal will include dynamic rotation for a time in one direction, followed by a pause, and then dynamic rotation for a time in the opposite direction. The period of dynamic rotation of the holder may be varied from 1 second to 20 seconds. Mechanical limitations resulting from the use of a stirring motor, switching relays, and the like, tend to prevent utilizing a period of rotation less than 1 second. On the other hand, rotation for periods substantially in excess of 20 seconds before reversal does not lead to a sufficient reduction in film thickness variation to warrant the use of the reversible rotation method.

During the time interval in which the holder is reversed from one direction to the opposite, growth of the film continues, but at a slower rate. In order to reduce any effects such growth might have on the final film, that time interval between reversals should be limited. Within the rotation range of 1 second to 20 seconds, a pause in time of up to 20 percent that of the rotation period does not lead to a substantial reduction of the plating rate of the solution, due to the continued dynamic conditions resulting from solution movement around the holder for the first few seconds after stopping the holder. For rotation times in excess of 10 seconds, however, it is preferred to use a pause of no more than 10 percent that of the rotation period to ensure that the plating rate does not substantially decrease during the pause.

Using the rotational velocity rates described earlier, a rotation in one direction for ten seconds followed by a one second pause and a rotation in the other direction for ten seconds, etc., has been shown to reduce nonuniformity of film thickness from about 30 percent to about 10 percent. A time period of rotation as short as 5 seconds in each direction with a A second pause between rotations has also been used with equal success.

In order to ensure as complete a uniformity of thickness as possible, it will be necessary to ensure that the surfaces to be plated are tangential to the circle defined by the holder at the center of the respective surfaces,

in order that the solution flow be approximately the same in both directions, as shown in FIG. 1. However, where periodic reversal of the substrate holder is not required, the surfaces need not be so arranged, but rather may be arranged in any fashion that exposes the surface to be plated to the solution, so long as the linear velocity requirements are met.

What is claimed is: l. A method for electroless deposition of a metal on a multiplicity of substrates mounted on a holder suspended in a solution, characterized in that a. the surfaces of the substrates to be plated are tangential to the circle defined by the holder at the center of the surfaces; b. the holder is rotated at a linear velocity which achieves a rate of about 50 centimeters per second to 300 centimeters per second at the substrates; and

c. the direction of rotation is periodically reversed,

with a time of rotation of from 1 second to 20 seconds and a maximum interval between reversals of 20 percent of said time of rotation.

2. The method of claim 1 in which the metal to be deposited consists essentially of gold.

3. The method of claim 2 in which gold is electrolessly deposited from an aqueous solution consisting essentially of KAu(CN in the concentration range of 0.001M to 0.05M, KCN in the concentration range of 0.005M to 0.5M, KOH in the concentration range of 0.05M to lM, and KBl-L in the concentration range of 0.05M to IM. 

2. The method of claim 1 in which the metal to be deposited consists essentially of gold.
 3. The method of claim 2 in which gold is electrolessly deposited from an aqueous solution consisting essentially of KAu(CN)2 in the concentration range of 0.001M to 0.05M, KCN in the concentration range of 0.005M to 0.5M, KOH in the concentration range of 0.05M to 1M, and KBH4 in the concentration range of 0.05M to 1M. 